Floating spar for supporting production risers

ABSTRACT

A subsea production system is provided for producing a number of subsea wells which may be arranged in groups. Each of the groups of subsea wellheads is connected to deliver production flow to a subsea manifold and each subsea manifold is connected to deliver production flow to a production riser. A plurality of production risers each being connected to receive production flow from one of said subsea manifolds extend from the subsea manifolds for groups of wells. A deep draft floating spar is located generally above the subsea wellheads with mooring lines and has a production platform located above the sea surface and has buoyancy and ballast chambers to control floatation. The spar structure defines a riser bore receiving the production risers extending from the subsea wellheads to the production platform. The spar is also capable of being shifted laterally by its mooring lines for positioning above a selected well to thus permit well intervention activities as needed. The subsea wells are each provided with wellheads having a removable cap to permit ROV actuated cap removal and replacement as needed to permit well intervention.

[0001] Applicants hereby claim the benefit of United States Provisionalapplication Ser. No. 60/092,354 which was filed on Jul. 10, 1998 by JohnA. Fitzgerald and Harold B. Skeels and entitled Floating Spar ForSupporting Production Risers, which Provisional Application isincorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a floating spar for supporting aproduction platform, and more particularly to such a floating spar forsupporting production risers extending from subsea manifolds to theproduction platform in deep water offshore wells.

[0004] 2. Description of the Prior Art

[0005] Oil and gas production spars currently utilize a number of subseawells placed a given lateral distance on the sea floor and connected tosurface facilities via individual risers where a Christmas tree isattached for well control. Wells for deepwater typically are very heavygiven their extended length and in some cases multiple barriers wheremultiple concentric casing riser joints exist. Since a production sparis a floating vessel, each riser must be vertically tensioned tomaintain its structural integrity. Hydraulic piston assemblies,electromechanical devices, and dashpots are some of the mechanisms usedto maintain a constant tension while the spar is heaving or movinglaterally (due to the ocean environmental forces). Buoyancy devicesattached to riser strings have also been used to allow the risers tofree stand independently of the spar's hull. This method is the mostadvantageous with respect to the spar since the tension created by thebuoyancy devices are not transferred to the spar hull, thereby freeingup the displacement of the spar's hull to support the weight of the sparand the facilities placed on top.

[0006] The drawback to this method is size. To make an offshoreproduction spar economically viable, several wells must be tied back tothe surface facility, each requiring a certain amount of space in thecenter of the spar for the riser and its buoyancy devices. As waterdepth increases, riser weight increases. As riser weight increases,space for buoyancy to hold up the riser increases. As the spaceincreases, so does the spar's hull diameter to accommodate the need foradded space. If the spar's hull is larger, it is more costly to buildand install, requiring more wells. Therefore a spar may reach aneconomic limit, simply because the water depth and number of wellscreate a spar hull so large as to make it uneconomical. Another aspectthat may increase riser weight or size is the concept of “barriers”. Ifa well's fluid control devices (tree and manifolds) are at the surface,there may be a requirement for extra conduits in the riser design forboth structural protection and pressure containment. Added conduits willincrease both size and weight to the riser.

[0007] U.S. Pat. No. 5,706,897 dated Jan. 13, 1998 is directed to afloating spar which is a deep-draft floating caisson of a hollowcylindrical construction and utilized primarily for deep water offshorewell operations at depths of 2,000 feet or more. The floating spar isanchored by mooring lines to the sea floor and may extend seven hundredfeet, for example, below the surface of the water. The spar or caissonshown in the '897 patent is directed primarily to a caisson for drillingrisers for supporting a high pressure drilling riser and a low pressuredrilling riser extending from a subsea wellhead. FIGS. 9 and 10,however, are directed to production risers in which a subsea tree isadded to provide a mechanical safety barrier at the sea floor. Above thesubsea tree is the vertical riser extending to a production manifold atthe surface. An additional surface tree is provided for fluid controlpurposes. Thus, a production riser extends from each subsea wellhead tothe surface location via a subsea tree, riser conduit, surface tree, andsurface manifold.

[0008] The utilization of individual production risers extending fromeach subsea wellhead through the spar to a surface manifold and surfacetree results in a substantial weight exerted on the spar particularlywhen multiple subsea wellheads, such as ten or more, are being utilizedfor product supply. Also, a substantial space within the spar or caissonis required for the multiple lines extending through the space to thesurface platform or deck. Floatation tanks within the spar are utilizedfor tensioning the risers. In some instances, the risers and wellheadconnector are deployed and recovered through the internal diameter ofthe buoys. The buoys must therefore be sized to permit the passage ofthe large diameter wellhead connector which normally controls theinternal diameter of the spar and contributes to the overall size of thespar.

[0009] It is desired that a spar be of a minimal size and weight forminimizing costs and simplifying construction, installation andoperation.

SUMMARY OF THE INVENTION

[0010] The present invention is directed to an offshore productionsystem utilizing a spar or caisson anchored to the sea floor by mooringlines and supporting a production platform above the sea level. Aplurality of subsea wellheads each has a subsea tree mounted thereonwith a removable tree cap to permit access to the subsea tree and subseawellhead. Production conduits from the annulus and production bores ofeach subsea tree extend to either: a production riser to the spar or asubsea manifold which receives conduits from multiple subsea trees, suchas five or ten subsea trees, for example. Subsea manifolds are normallyprovided, particularly when a plurality of the sub sea wells are locatednearby each other to reduce the number of conduits extending to asurface location. Production risers from subsea trees and/or manifoldsextend from the sea floor through the spar to the production platform ontop of the spar. Also, test lines and umbilical lines may extend fromthe subsea trees and manifolds through the spar to the productionplatform for flow control, test or maintenance work. The productionrisers from the subsea tree and manifolds may be flexible cables orvertical centenary risers and formed of various materials.

[0011] To intervene or provide access to the subsea tree, such as thetubing string, the spar may be positioned over the designated well withthe intervention riser system over the tree. The tree cap is thenremoved and the intervention system is then landed and locked onto thetop of the tree thereby permitting intervention in the well. To minimizeintervention hardware weight and the number of trips that equipment hasto travel between the surface and the sea floor, the subsea trees mayutilize a light weight tree cap which may be deployed and recovered by aremotely operated vehicle (ROV).

[0012] Utilizing subsea technology, the costs of deepwater spars arereduced by reducing the number of risers between the sea floor and thespar. Instead of individual risers for each well, the wells arecompleted in a standard subsea configuration which are subsequently sentto the surface individually via a light weight minimal barrier riser, orco-mingled together via manifolding on the sea floor and sent to thesurface by a single larger bore riser to the spar facility. Theproduction riser(s) may be vertically supported in the same manner asindividual well risers. The production riser itself may be larger indiameter than the individual well riser, requiring bigger buoyancy tosupport its weight. Other risers for pipeline pigging, well testing, andcontrol (electrical/hydraulic line) cables to operate the subsea wellsmay also be needed, but the overall number of suspended conduits fromthe spar is drastically reduced for the same number of wells. The fewernumber of conduits required results in a smaller space and spar hullsize requirement; leading to lower spar hull fabrication costs. Subseamulti-well technology also does not limit the number of wells needed,nor the structural and geometric problems of a riser associated with thelateral reach out to outlying wells. In addition, single subsea wellswith a subsea tree leading to a production pipeline/riser conduit act asboth the safety barrier and flow control are a simpler design and a morecost effective approach to the subsea safety tree and surface tree oneither end of the spar riser configuration.

[0013] The reduced area for risers also lets the spar better utilize itsdeck space and displacement capacity for drilling and workover derricks,subsea risers and subsea blowout preventers. With fewer risers, the sparmay move about on its anchor mooring spread to position itself over anywell for subsea drilling completion or workover operations permittingtubing intervention into individual subsea wells.

[0014] It is an object of this invention to provide a deep-draftfloating spar of minimum size and weight for supporting productionrisers extending from subsea manifolds to a production platform on thespar.

[0015] A further object of this invention is to provide such a subseaproduction system utilizing subsea trees which have a removable tree capfor intervention and access to the subsea well without necessarily goingthrough the production riser. Small intervention well control hardwarecan be run and suspended from the spar for periodic maintenance andworkovers.

[0016] Another object of the invention is the provision of such a sparsubsea production system in which subsea trees have production pipelinesextending to subsea manifolds which, in turn, have production risersextending from the manifolds through the spar to the production platformthereby eliminating surface trees and minimizing any surface manifoldsfor the production platform.

[0017] Other objects, features, and advantages of the invention will bemore apparent from the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic view of a floating spar production systemincluding a production platform supported on a buoyant spar with productrisers extending from subsea manifolds (or subsea trees) through adeep-draft caisson spar to the production platform; and

[0019]FIG. 2 is a schematic view of a subsea tree connected to a subseawellhead and having a removable tree cap for removal by a remotelyoperated vehicle (ROV) to permit access to the subsea tree and subseawellhead such as may be required for workover operations or the likeusing lightweight intervention techniques.

DESCRIPTION OF THE INVENTION

[0020] Referring to the drawings a floating spar or caisson is generallyindicated at 10 having a production platform 12 with a plurality ofdecks mounted thereon above the sea level 11. Spar 10, for example, maybe about 700 feet in length and about 75 feet in diameter, with thewater depth over about 2000 feet. Mooring lines 14 are secured to anchorpiles (not shown) on sea floor 16 for anchoring of spar 10. Six (6) oreight (8) mooring lines 14 are preferably utilized for mooring of spar10. Buoys which comprise buoyancy tanks or chambers 18 are mountedwithin spar 10 along with ballast chambers 20. An axial bore or slot 22is provided in spar 10 through buoyancy tanks 18 and ballast chambers 20to receive a plurality of production risers 24, 26, 28. Test andumbilical lines may also be provided within spar 10. Suitable supportmembers 30 on spar 10 within riser bore 22 support production risers 24,26 and 28.

[0021] Mounted on sea floor 16 are a plurality of subsea wellheads 36.Each subsea wellhead 36 has a subsea tree 38 connected thereto with asuitable connector and an upper removable tree cap 40 is provided oneach subsea tree 38. A horizontal subsea tree having a removable treecap which is satisfactory may be purchased from the FMC Corporation,Petroleum Equipment and Systems Division, of Houston, Tex. Subsea tree38 is preferable of a dual bore type. Production and annulus conduits42, 44 extend from each subsea tree 38 to an associated dual bore subseamanifold 46, 48 or 50 on sea floor 16. Riser 42 extends from the tubingstring of the well, while riser 44 extends from the annulus of the well.Production risers 24, 26 and 28 from respective subsea manifolds 46, 48and 50 extend upwardly through riser slot 22 in spar 10 to a surfacemanifold 52 on production platform 12. Suitable riser supports 30 inslot 22 support production risers 24, 26 and 28. Suitable test lines andelectrical/hydraulic umbilicial lines (not shown) may extend to thesubsea manifolds and subsea trees for testing and control as needed.

[0022] Spar 10 may be moved as much as about 250 feet in any directionwithout disconnecting mooring lines 14 from spar 10. Each subseawellhead 36 and subsea tree 38 having a removable tree cap 40 thereon isarranged so that full vertical access and workovers may be obtained byremoval of the tree cap 40 without removing the subsea tree. It isnecessary for various reasons to intervene into the tubing string of asubsea well from time to time, such as might be required for shiftingsleeves, wax cutting, bottom hole pressure surveys, and bailing sand,for example. Wire line or coiled tubing may be utilized in anintervention riser system for intervening into the subsea well. Theparticular type of intervention riser system depends on various factors,such as water depth, well pressure, currents, spar length, and may beconstructed of a composite material or coiled tubing.

[0023] The spar 10 is first positioned vertically over the subsea tree38 as shown in FIG. 2. A remotely operated vehicle (ROV) illustratedgenerally at 54 is normally utilized with the intervention riser system.Subsea tree cap 40 is first removed utilizing the ROV. An interventionsystem (not shown) is landed and locked onto the top of tree 38. Thetree cap 40 is normally provided with a space for positioning of ROV 54over cap 40 in an aligned position for removal of cap 40 and landing andlocking of the intervention system onto tree 38. After the completion ofthe workover or other operation, ROV 54 picks up and reinstalls tree cap40 and tests the connection to insure pressure integrity.

[0024] The production risers 24, 26, 28 (FIG. 1) extending through spar10 may be tensioned, if needed, by buoys 18 within spar 10 or by pistontype tensioners as well known. For further details of spar 10, theentire disclosure of U.S. Pat. No. 5,706,897 is incorporated byreference. ROV 54 may be controlled from platform 12 or a separate divesupport vessel.

[0025] While three manifolds 46, 48 and 50 are illustrated with eachmanifold having a separate production riser extending to platform 12, itmay be desirable to have only a single manifold with a single productionriser extending to surface platform 12. Also, it may be desirable tocombine production risers 24, 26 and 28 into a single riser extending tosurface platform 12 through spar 10 as less space in spar 10 could beutilized.

[0026] In the present invention, a floating spar production systemutilizes subsea trees having ROV removable tree caps and connected byrisers to subsea manifolds which, in turn, have production risersextending from the subsea manifolds through the spar to the productionplatform. Such a system results in a spar of minimal size and weight andeach subsea tree having a removable tree cap thereon is adapted forvertical access for workover or other operations.

[0027] In view of the foregoing it is evident that the present inventionis one well adapted to attain all of the objects and featureshereinabove set forth, together with other objects and features whichare inherent in the apparatus disclosed herein.

[0028] As will be readily apparent to those skilled in the art, thepresent invention may easily be produced in other specific forms withoutdeparting from its spirit or essential characteristics. The presentembodiment is, therefore, to be considered as merely illustrative andnot restrictive, the scope of the invention being indicated by theclaims rather than the foregoing description, and all changes which comewithin the meaning and range of equivalence of the claims are thereforeintended to be embraced therein.

We claim:
 1. A method for selectively producing and conductingintervention operations on a plurality of subsea wells having subseawellheads located at the sea bed, comprising: (a) mooring a deep draftfloating spar generally above the subsea wellheads with mooring lines,said deep draft floating spar having a production platform located abovethe sea surface, having buoyancy and ballast chambers and defining ariser bore receiving at least one production riser extending from thesubsea wellheads to said production platform; (b) producing the subseawells through said at least one production riser while permitting arange of lateral movement of said floating spar responsive to externalforces of water current, wind and the like; (c) for intervention withrespect to a selected well, shifting said floating spar to a stationabove the selected subsea wellhead; and (d) conducting well interventionoperations on the selected well.
 2. The method of claim 1 , wherein eachof said subsea wellheads has a removable wellhead cap for permittingwell intervention, said method comprising: (a) prior to wellintervention, removing said removable wellhead cap; (b) conducting saidwell intervention operations; and (c) replacing said removable wellheadcup after completion of said well intervention operations.
 3. The methodof claim 2 , wherein a remote operated vehicle (ROV) is provided forremoval and replacement of removable wellhead caps, said methodcomprising: (a) actuating said ROV for removal of said removablewellhead cap from the selected wellhead; and (b) after completing saidwell intervention operation, actuating said ROV for replacing saidremovable wellhead cap to permit resumption of well production.
 4. Themethod of claim 1 , wherein said subsea wellheads are arranged ingroups, with each of said groups having a subsea manifold connected toreceive production flow from each of the wellheads of said group andsaid subsea manifold having a production riser extending through saidriser bore, said method comprising: (a) with said production riserproducing from at least one of the wellheads of said group of wellheadsthrough at least one of said subsea manifolds, shifting said deep draftfloating spar laterally to a station above a selected wellhead; and (b)conducting well intervention operations through the selected wellheadwhile continuing said producing from at least one of the wellheads ofsaid group of wellheads through at least one of said subsea manifolds.5. The method of claim 1 , wherein said subsea wellheads are arranged ingroups, with each of said groups having a subsea manifold connected toreceive production flow from each of the wellheads of said group andeach of said subsea manifolds of said groups having a production riserextending through said riser bore, said method comprising: (a) with saidproduction risers producing from at least one of the wellheads of eachof said group of wellheads through said subsea manifold of said group,shifting said deep draft floating spar laterally to a station above aselected wellhead designated for intervention; and (b) conducting wellintervention operations through the selected wellhead while continuingsaid producing from the wellheads of said group of wellheads throughsaid subsea manifolds.
 6. A subsea production system for a plurality ofsubsea wells each having subsea wellheads located at the sea floor,comprising: (a) a deep draft floating spar adapted for locationgenerally above the subsea wellheads and having a production platformlocated above the sea surface, having buoyancy and ballast chambers anddefining a riser bore; (b) mooring lines for mooring said deep draftfloating spar and for controlling lateral positioning of said deep draftfloating spar for stationing thereof above a selected wellhead intendedfor intervention; (c) at least one subsea production manifold connectedto receive production from a plurality of said wellheads; and (d) atleast one production riser being connected to said at least one subseaproduction manifold and extending upwardly from said at least one subseaproduction manifold through said riser bore to said production platform.7. The subsea production system of claim 6 , comprising: (a) said subseawells being arranged in groups; (b) said subsea production manifoldseach being connected to receive production flow from the wellheads ofone of said groups of wellheads; and (c) said at least one productionriser being a plurality of production risers each being connected toreceive production flow from one of said subsea manifolds and extendingfrom said subsea manifold through said riser bore and to said productionplatform.
 8. The subsea production system of claim 7 , comprising: (a)said plurality of subsea wells each having a removable cap, beingremovable to permit well intervention activities; and (b) said removablecap being removable and replaceable by ROV controlled servicingactivities.
 9. The subsea production system of claim 7 , comprising: (a)said plurality of subsea wells defining groups of wells, each grouphaving two or more wells each having a wellhead; and (b) a subseamanifold being connected in production flow receiving relation with saidwellheads of a group of wells and having one of said production risersconnected in flow receiving relation therewith.
 10. The subseaproduction system of claim 9 , comprising: said subsea manifolds beingdual bore subsea manifolds.
 11. The subsea production system of claim 9, comprising: (a) said subsea manifolds being dual bore subseamanifolds; and (b) said plurality of wellheads having production andannulus conduits for production and which are connected for delivery ofproduction fluid to the dual bore subsea manifold for the group ofwells.
 12. The subsea production system of claim 6 , comprising: (a)said plurality of subsea wells being located over a defined area of theseabed; and (b) said deep draft floating spar having a diameter lessthan said defined area of said seabed and adapted to be laterallyshifted for positioning directly above any selected one of saidplurality of subsea wells.
 13. A subsea production system comprising:(a) a plurality of subsea wells each having subsea wellheads located atthe sea floor and being located on a defined area of the sea floor (b) adeep draft floating spar adapted for location generally above the subseawellheads and having a production platform located above the seasurface, having buoyancy and ballast chambers and defining a riser bore,said deep draft floating spar having a diameter less than said definedarea of the sea floor; (c) a plurality of mooring lines for mooring saiddeep draft floating spar and for controlling lateral positioning of saiddeep draft floating spar for stationing thereof above a selectedwellhead intended for intervention; (d) a plurality of subsea productionmanifolds each being connected to receive production from a group ofsaid plurality of wellheads; and (e) a plurality of production riserseach being connected to one of said subsea production manifolds andextending upwardly through said riser bore to said production platform.14. The subsea production system of claim 13 , comprising: (a) saidsubsea wellheads being arranged in groups; (b) said subsea productionmanifolds each being connected to receive production flow from thewellheads of one of said groups of wellheads; and (c) said at least oneproduction riser being a plurality of production risers each beingconnected to receive production flow from one of said subsea manifoldsand extending from said subsea manifold through said riser bore and tosaid production platform.
 15. The subsea production system of claim 13 ,comprising: (a) said plurality of subsea wells each having a removablecap, being removable to permit well intervention activities; and (b)said removable cap being removable and replaceable by ROV controlledservicing activities.
 16. The subsea production system of claim 13 ,comprising: (a) said plurality of subsea wells defining groups of wells,each group having two or more wells each having a wellhead; and (b) asubsea manifold being connected in production flow receiving relationwith said wellheads of a group of wells and having one of saidproduction risers connected in flow receiving relation therewith
 17. Thesubsea production system of claim 16 , comprising: said subsea manifoldsbeing dual bore subsea manifolds.
 18. The subsea production system ofclaim 16 , comprising: (a) said subsea manifolds being dual bore subseamanifolds; and (b) said plurality of wellheads having production andannulus conduits for production and which are connected for delivery ofproduction fluid to the dual bore subsea manifold for the group ofwells.
 19. The subsea production system of claim 13 , comprising: (a)said plurality of subsea wells being located over a defined area of theseabed; and (b) said deep draft floating spar having a diameter lessthan said defined area of said seabed and adapted to be laterallyshifted for positioning directly above any selected one of saidplurality of subsea wells.